Re: Error when using @container_of

[Giovanni Gherdovich <ggherdovich@suse.cz>]

On Mon, 2019-04-22 at 12:06 +0200, Giovanni Gherdovich wrote:
> Hello,
> 
> when I use the macro @container_of in systemtap I'm getting an error I don't
> understand.
> 
> This is my tapset:
> 
>     #!/root/systemtap-latest/bin/stap
> 
>     probe kernel.function("__update_load_avg_se") {
>             printf("%x\n", @container_of($se, "struct task_struct", se));
>             exit();
>     }
> 
> and this is the error:
> 
>     # ./repro.stp
>     semantic error: 'struct task_struct' (./include/linux/sched.h:602) is being
>     accessed instead of a member such as '->acct_rss_mem1': operator '@cast' at
>     /root/systemtap-latest/share/systemtap/tapset/container_of.stpm:2:5
> 
>             source:     @cast(@ptr - @offsetof(@type, @member), @type)
>                         ^
>             in expansion of macro: operator '@container_of' at ./repro.stp:4:17
>             source:         printf("%x\n", @container_of($se, "struct task_struct", se));
>                                            ^
> 
>     Pass 2: analysis failed.  [man error::pass2]

Hi again,

I'm posting here for future reference: I've been explained that my mistake was
to not use the address-of operator '&' after @container_of. Since my original
goal was to get the PID of a sched entity, the snipped above can be fixed
with:

    probe kernel.function("__update_load_avg_se") {
	    p = &@container_of($se, "struct task_struct", se);
	    printf("pid: %d\n", p->pid);
	    exit();
    }

or even by chaining a field access with -> without using '&' like:

    probe kernel.function("__update_load_avg_se") {
	    printf("pid: %d\n", @container_of($se, "struct task_struct", se)->pid);
	    exit();
    }

In hindsight I should say that the error message makes perfect sense: "you're
accessing the struct itself, you should access a field instead!". What I missed
is that I had to take the address of the struct.


Regards,
Giovanni Gherdovich